The Home Office named 16 April as the date when the mephedrone ban will come into effect, as websites selling the drug today started to close down while promising to honour outstanding orders.

Liberal Democrat MPs warned ministers that there were legal question marks over their rush to ban mephedrone, or meow meow as it is known, which could jeopardise future prosecutions of dealers.

Ministers also clarified that the class B status of mephedrone and related cathinone compounds would not extend to khat, the natural plant from which they are derived and which is widely used as a stimulant by Britain's Somali community.

Dr Evan Harris, the Liberal Democrat MP for Oxford West and Abingdon, said the six resignations from the government's advisory council on the misuse of drugs had left it inquorate and legally unable to issue formal advice to the home secretary.

He also questioned how a decision to ban mephedrone without first publishing the council's report on its potential harms complied with new government guidelines on the treatment of scientific advice.

He was backed up by the Liberal Democrat home affairs spokesman, Chris Huhne, who also questioned how confident ministers were that the ban was being implemented in a "lawful manner and cannot be challenged in the courts".

But home office minister David Hanson, answering an urgent question in the Commons, said he was sure that the resignations, including that of Dr Polly Taylor, a consultant vet, on Sunday night had "no material effect on the consideration of these issues". He said that Taylor had "had the opportunity to play her part".

His response is unlikely to end doubt over the matter as the 1971 Misuse of Drugs Act says that a home secretary cannot ban a substance without first taking formal advice from the advisory council on the misuse of drugs.

Harris said yesterday there were three vacancies currently on the council including positions for a pharmacist and dentist as well as Taylor's position representing veterinary science.

The statutory instrument scheduling mephedrone as a class B drug with a maximum prison sentence of 14 years available for importing and supply was published yesterday. Both the Conservatives and the Liberal Democrats gave it their formal backing.

It is expected that the Commons will debate the order implementing the ban next Thursday – expected to be the last day before Parliament is dissolved for the general election.